Sometimes the player's screen looks down quickly when the screen is being touched.
When breaking blocks or punching (whether mobs or otherwise), the screen can suddenly glitch and the player can find himself/herself looking directly downwards. This can create major issues when building, while fighting mobs, or during PvP.
Ugh... really it still happens?
Then it's a duplicate of MCPE-18898 which is believed to be fixed already.